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An insecticide is a pesticide used against insects in all developmental forms. They include ovicides and larvicides used against the eggs and larvae of insects respectively. Insecticides are used in agriculture, medicine, industry and the household. The use of insecticides is believed to be one of the major factors behind the increase in agricultural productivity in the 20th century. Nearly all insecticides have the potential to significantly alter ecosystems; many are toxic to humans; and others are concentrated in the food chain. It is necessary to balance agricultural needs with environmental and health issues when using insecticides.

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Systemic insecticides are incorporated by treated plants. Insects ingest the insecticide while feeding on the plants.

Contact insecticides are toxic to insects brought into direct contact. Efficacy is often related to the quality of pesticide application, with small droplets (such as aerosols) often improving performance.

Natural insecticides, such as nicotine and pyrethrum, are made by plants as defences against insects. Nicotine based insecticides have been barred in the U.S. since 2001 to prevent residues from contaminating foods. [1]

Inorganic insecticides are manufactured with metals and include arsenatescopper- and fluorine compounds, which are now seldom used, and sulfur, which is commonly used.

Organic insecticides are synthetic chemicals which comprise the largest numbers of pesticides available for use today.

Mode of action -- how the pesticide kills or inactivates a pest -- is another way of classifying insecticides. Mode of action is important in predicting whether an insecticide will be toxic to unrelated species such as fish, birds and mammals.

Heavy metals, e.g. lead, mercury, arsenic, as well as plant toxins such as nicotine have been used for many years. Various plants have been used as folk insecticides for centuries, including tobacco and pyrethrum. Some farmers are reporting successfully using spray of crudely fermented alcohol as an effective insecticide.

With the rise of the modern chemical industry it was possible to form organochlorides, such as chlorinated hydrocarbons. The organochlorides used in chemical warfare tend to be more potent electrophiles than those used as insecticides. For instance mustard gas (sulfur mustard, HD) is a potent alklating agent which uses neighbouring group participation of the sulfur to make the alkyl chloride a stronger electrophile. It is likely that the chlorine in DDT is used to tune the lipophilicity of the compound, and to alter the shape and electrostatic effects involved in the interactions of the insecticide and the biomolecules in the target organism. For instance DDT works by opening the sodium channels in the nerve cells of the insect.

The next large class was the organophosphates, both the insecticides and the chemical warfare agents (such as sarin, tabun, soman and VX) work in the same way. All these compounds bind to acetylcholinesterase and other cholinesterases. This results in disruption of nervous impulses, killing the insect or interfering with its ability to carry on normal functions. Carbamate insecticides have similar toxic mechanisms but have a much shorter duration of action and are thus somewhat less toxic.

Organophosphates have an additive toxic effect to wildlife, so multiple exposures to the chemicals amplifies the toxicity.[1]

To mimic the insecticidal activity of the natural compound pyrethrum another class of pesticides, pyrethroid pesticides, have been developed. These are nonpersistent and much less acutely toxic than organophosphates and carbamates.

Recent efforts to reduce broad spectrum toxins added to the environment have brought biological insecticides back into vogue. An example is the development and increase in use of Bacillus thuringiensis, a bacterial disease of Lepidopterans and some other insects. It is used as a larvicide against a wide variety of caterpillars. Because it has little effect on other organisms, it is considered more environmentally friendly than synthetic pesticides. The toxin from Bacillus thuringiensis (Bt toxin) has been incorporated directly into plants through the use of genetic engineering. Other biological insecticides include products based on entomopathogenic fungi (e.g. Metarhizium anisopliae), nematodes (e.g. Steinernema feltiae) and viruses (e.g. Cydia pomonella granulovirus).

Some insecticides kill or harm other creatures in addition to those they are intended to kill. For example, birds may be poisoned when they eat food that was recently sprayed with insecticides or when they mistake insecticide granules on the ground for food and eat it.[1]

Sprayed insecticides may drift from the area to which it is applied and into wildlife areas, especially when it is sprayed aerially.[1]

One of the bigger drivers in the development of new insecticides has been the desire to replace toxic and irksome insecticides. DDT was introduced as a safer alternative to the lead and arsenic compounds. It is the case that when used under the correct conditions that almost any chemical substance is 'safe', but when used under the wrong conditions most insecticides can be a threat to health and/or the environment.

Some insecticides have been banned due to the fact that they are persistent toxins which have adverse effects on animals and/or humans. An oft-quoted case is that of DDT, an example of a widely used (and maybe misused) pesticide, which was brought to public attention by Rachel Carson's book, Silent Spring. One of the better known impacts of DDT is to reduce the thickness of the egg shells on predatory birds. The shells sometimes become too thin to be viable, causing reductions in bird populations. This occurs with DDT and a number of related compounds due to the process of bioaccumulation, wherein the chemical, due to its stability and fat solubility, accumulates in organisms' fatty tissues. Also, DDT may biomagnify which causes progressively higher concentrations in the body fat of animals farther up the food chain. The near-worldwide ban on agricultural use of DDT and related chemicals has allowed some of these birds--such as the peregrine falcon--to recover in recent years. A number of the organochlorine pesticides have been banned from most uses worldwide and globally they are controlled via the Stockholm Convention on persistent organic pollutants. These include: aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, mirex and toxaphene.

While the overuse of DDT led to a reduction in its use, opponents of traditional environmentalism often cite it as an example of environmentalism going too far and interfering with malaria eradication, even estimating the cost of human lives resulting from the DDT ban; for instance the novelist Michael Crichton states in his bestselling book, State of Fear:

"Since the ban, two million people a year have died unnecessarily from malaria, mostly children. The ban has caused more than fifty million needless deaths. Banning DDT killed more people than Hitler."

This accusation, while sensational, is erroneous, as no ban exists on the use of DDT for eradication of malaria or any other mosquito borne disease.[2] Groups fighting malaria have praised the ban on agricultural use of DDT, since it reduces the rate with which mosquitoes become resistant to DDT, which is the main reason it is not used more often to fight malaria:

"The outcome of the treaty is arguably better than the status quo going into the negotiations over two years ago. For the first time, there is now an insecticide which is restricted to vector control only, meaning that the selection of resistant mosquitoes will be slower than before."[2]

According to Agricultural production and malaria resurgence in Central America and India, Chapin, Georgeanne & Robert Wasserstrom, Nature, Vol. 293, 1981, page 183), the lives actually saved due to banning agricultural use of DDT can be estimated:

"Correlating the use of DDT in El Salvador with renewed malaria transmission, it can be estimated that at current rates each kilo of insecticide added to the environment will generate 105 new cases of malaria."

Other insecticides have had to be used as an alternative to DDT because the population of insects have become resistant to DDT. Most of the newer insecticides are more specific in their actions and are designed to break down into non-toxic components within a few days of application. Nonetheless, misuse of insecticides remains an environmental and economic issue. For example, in Bangladesh most of the insecticide applications by rice farmers are apparently unnecessary.[3]

Integrated pest management or IPM in the home begins with restricting the availability to insects of three vital commodities: shelter, water and food. If insects become a problem despite such measures, IPM seeks to control them using the safest possible methods, targeting the approach to the particular pest.

Insect repellent referred to as "bug spray", comes in a plastic bottle or aerosol can. Applied to clothing, arms, legs, and other extremities, the use of these products will tend to ward off nearby insects. This is not an insecticide.

Insecticide used for killing pests—most often insects, and arachnids—primarily comes in an aerosol can, and is sprayed into the air or a nest as a means of killing the animal. Fly sprays will kill house flies, blowflies, ants, cockroaches and other insects and also spiders. Other preparations are granules or liquids that are formulated with bait that is eaten by insects. For many household pests bait traps are available that contain the pesticide and either pheromone or food baits. Crack and crevice sprays are applied into and around openings in houses such as baseboards and plumbing. Pesticides to control termites are often injected into and around the foundations of homes.

Active ingredients of many household insecticides include permethrin and tetramethrin, which act on the nervous system of insects and arachnids.

Bug sprays should be used in well ventilated areas only, as the chemicals contained in the aerosol and most insecticides can be harmful or deadly to humans and companion animals. All insecticide products including solids, baits and bait traps should be applied such that they are out of reach of wildlife, companion animals and children.